LOve the picture, but Firefly isn't Rainbows Mother because in the song The rappin' Hist'ry of the Wonderbolts, Pinkie Pie says: A general named Firefly, witch means that Firefly wasn't Rainbow dash's mother because she was around in ancient times. But still a cute little picture.
The standard mare base is used for teen and older ponies. Granny smith has got the oldest base (she is old) so there is a possibility that they are the same-ish age but I believe not. So I still think they are mother and daughter, they where standing pretty close in that episode with the flashback of her on top of her farther (really rubbish with episode names sorry ^^')